Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving member. The ink may be stored in reservoirs that are located within cartridges installed in the printer. Such ink may be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink may be in the form of pellets, ink sticks, granules, pastilles, or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device, which melts the solid ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. Other inkjet printers use gel ink. Gel ink is provided in gelatinous form, which is heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead. Once the melted solid ink or the gel ink is ejected onto the image receiving member, the ink returns to a solid, but malleable form, in the case of melted solid ink, and to gelatinous state, in the case of gel ink.
A typical inkjet printer uses one or more printheads with each printhead containing an array of individual nozzles through which drops of ink are ejected by inkjets across an open gap to an image receiving surface to form an ink image during printing. The image receiving surface may be the surface of a continuous web of recording media, a series of media sheets, or the image receiving surface may be a rotating surface, such as the surface of a rotating print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an aperture, usually called a nozzle, in a faceplate of the printhead. The actuators expel an ink drop in response to an electrical signal, sometimes called a firing signal. The magnitude, or voltage level, of the firing signals affects the amount of ink ejected in an ink drop. The firing signal is generated by a printhead controller with reference to image data. A print engine in an inkjet printer processes the image data to identify the inkjets in the printheads of the printer that must be operated to eject a pattern of ink drops at particular locations on the image receiving surface to form an ink image corresponding to the image data. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to electronic image data.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads are registered with reference to the image receiving surface and with the other printheads in the printer. Registration of printheads refers to a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the relative positions of the printheads with reference to the imaging surface and with reference to the other printheads in the printer. Operating the printheads in a printer to eject ink in correspondence with image data presumes that the printheads are level with one another across a width of the image receiving member and that all of the inkjets in the printhead are operational. The presumptions regarding the positions of the printheads, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the printheads cannot be verified, the analysis of the printed image should generate data that can be used either to adjust the printheads so they better conform to the presumed conditions for printing or to compensate for the deviations of the printheads from the presumed conditions.
Two or more printheads can be mounted linearly, or in other configurations, to a support structure, to form an array of printheads. Not only is registration between individual printheads important, but control of the registration of the supporting structure with respect to the image receiving surface is also desirable. The gap, or distance between support structure and the imaging surface, is controlled to optimize the imaging process. If the gap is too small, burnishing of the printheads can occur when the image receiving surface contacts the face of the printheads. Burnishing not only reduces the life of the printheads, but results in poor image quality and increased downtime of the printer during maintenance. If the gap it too large, image quality suffers, particularly in high speed printers, where a large gap can result in the ink drops being deposited at unintended locations.
The setting of a proper gap is important where a printer is designed to accept a variety of imaging surfaces, including surfaces having a tendency to wrinkle, having different thicknesses, or having uneven surfaces. Likewise, where a printer is used in an industry, such as advertising, where paper types and thickness can change from print job to print job, the capability of setting the gap between print jobs can be particularly important. In some instances, the location of the printer and its surrounding environment or the use of different types of inks necessitates setting the gap accordingly. Consequently, detecting the gap distance of the printhead array from the imaging surface and adjusting the gap distance appropriately are important considerations for image quality and printer operation.